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Efficient Algorithms for the Longest Path Problem

- Ryuhei UEHARA (JAIST)
- Yushi UNO (Osaka Prefecture University)

The Longest Path Problem

- Finding a longest (vertex disjoint) path in a

given graph - Motivation (comparing to Hamiltonian path)
- Approx. Algorithm, Parameterized Complexity
- More practical/natural
- More difficult(?)

The Longest Path Problem

- Known (hardness) results
- We cannot find a path of length n-ne in a given

Hamiltonian graph in poly-time unless PNP

Karger, Motwani, Ramkumar 1997 - We can find O(log n) length path Alon, Yuster,

Zwick1995 - (?O((log n/loglog n)2) Björklund, Husfeldt

2003) - Approx. Alg. achieves O(n/log n) AYZ95
- (?O(n(loglog n/log n)2)BH03)
- Exponential algorithm Monien 1985

The Longest Path Problem

- Known polynomial time algorithm
- Dijkstras Alg.(196?)Linear alg. for finding a

longest path in a tree

The Longest Path Problem

- Known polynomial time algorithm
- Dijkstras Alg.(196?)Linear alg. for finding a

longest path in a tree

The Longest Path Problem

- Known polynomial time algorithm
- Dijkstras Alg.(196?)Linear alg. for finding a

longest path in a tree

The Longest Path Problem

- Known polynomial time algorithm
- Dijkstras Alg.(196?)Linear alg. for finding a

longest path in a tree

Approaches to the Efficient Algs to Longest Path

Problem

- Extension of the Dijkstras algorithm
- Weighted trees (linear), block graphs (linear),

cacti (O(n2)). - Graph classes s.t. Hamiltonian Path can be found

in poly time - Some graph classes having interval

representations - (bipartite permutation, interval biconvex

graphs) - Dynamic programming to the graph classes that

have tree representations (on going) - Cacti(linear),

(ISAAC 2004)

(ISAAC 2004)

Approaches to the Efficient Algs to Longest Path

Problem

- Extension of the Dijkstras algorithm
- Weighted trees (linear), block graphs (linear),

cacti (O(n2)). - Graph classes s.t. Hamiltonian Path can be found

in poly time - Some graph classes having interval

representations - (bipartite permutation, interval biconvex

graphs) - Dynamic programming to the graph classes that

have tree representations (on going) - Cacti(linear),

(ISAAC 2004)

(ISAAC 2004)

1. Ex of Dijkstras Alg

- Bulterman et.al. (IPL,2002) showed that
- the correctness of Dijkstras alg stands for
- For each u,v,
- length of the shortest path between u and v

- length of the longest path between u and v
- For each u,v,w,
- d(u,v) ? d(u,w) d(w,v)
- For each u,v,w,
- d(u,v) d(u,w) d(w,v) if and only if
- w is on the unique path between u and v

1. Ex of Dijkstras Alg

- Construct G(V,E) from G(V,E) s.t.
- V?V
- For each u,v?V,
- length of the shortest path between u,v on G
- length of the longest path between u,v on

G - For each u,v?V,
- the shortest path between u,v on G is unique

1. Ex of Dijkstras Alg

- Theorem ExDijkstra finds a longest path if G and

G satisfy the conditions. - ExDijkstra G(V,E) and G(V,E)
- pick any vertex w in V
- find x?V with maxd(w,x) on G
- find y?V with maxd(x,y) on G
- x and y are the endpoints of the longest path in

G, and d(x,y) on G is its length.

1. Ex of Dijkstras Alg (Summary)

- Theorem Vertex/edge weighted tree (linear)
- Theorem Block graph (O(VE))
- Theorem Cactus (O(V2))

1. Ex of Dijkstras Alg (Cacti)

- Cactus
- Each block is a cycle
- Two cycle share at most one vertex which is a

separator

G

G

cactus

The longest path between u and v on G

The shortest path between u and v on G

1. Ex of Dijkstras Alg (Cacti)

- Sample

1. Ex of Dijkstras Alg (Cacti)

- Sample

1. Ex of Dijkstras Alg (Cacti)

- Sample

1. Ex of Dijkstras Alg (Cacti)

- Sample

1. Ex of Dijkstras Alg (Cacti)

- Sample

1. Ex of Dijkstras Alg (Cacti)

- Sample

1. Ex of Dijkstras Alg (Cacti)

- Sample

Graph classes s.t. Hamiltonian Path can be found

in poly time

- Fact 1
- Hamiltonian Path is NP-hard on a chordal graph.

(In fact, strongly chordal split

graphMüller,1997.) - Fact 2
- Hamiltonian Path is solvable on an interval graph

in linear time. Damaschke, 1993. - Our goal
- Poly-time algorithm for Longest Path on an

interval graph.

Interval Graphs

- An interval graph G(V,E) has an interval

representation s.t. u,v?E iff IunIv?f

Interval Graphs

- An interval graph G(V,E) has an interval

representation s.t. u,v?E iff IunIv?f

Iv

v

Iu

Hamiltonian Path linear time solvable.

u

Longest Path ????

? Restricted interval graphs

Restricted Interval Graphs

- An interval biconvex graph G(S?Y,E) has an

interval representation s.t

Y biconvex

s1

s3

s6

s8

s2

s4

s7

s5

S integer points

Restricted Interval Graphs

- Interval biconvex graph G(S?Y,E) is introduced

Uehara, Uno 2004 from graph theoretical

viewpoints

- Natural analogy of biconvex graphs (bipartite

graph class) - Generalization of proper interval graphs
- Generalization of threshold graphs
- Best possible class longest path can be found in

poly time

Poly-time alg for longest path on an interval

biconvex graph (idea)

- Find the trivial longest path P on GY
- Embed the vertices in S into P as possible
- Adjust endpoints if necessary.

3

4

3

7

1

9

2

3

8

8

30

Poly-time alg for longest path on an interval

biconvex graph (idea)

- Find the trivial longest path P on GY
- Embed the vertices in S into P as possible
- Adjust endpoints if necessary.

3

4

3

7

1

9

2

3

8

8

30

Poly-time alg for longest path on an interval

biconvex graph (idea)

- Find the trivial longest path P on GY
- Embed the vertices in S into P as possible
- Adjust endpoints if necessary.

3

4

7

9

8

8

30

3

4

3

7

1

9

2

3

8

8

Poly-time alg for longest path on an interval

biconvex graph (idea)

- Find the trivial longest path P on GY
- Embed the vertices in S into P as possible
- Adjust endpoints if necessary.

3

4

7

9

8

30

30

3

4

3

7

1

9

2

3

8

8

Poly-time alg for longest path on an interval

biconvex graph (idea)

- Find the trivial longest path P on GY
- Embed the vertices in S into P as possible
- Adjust endpoints if necessary.

3

- How can we determine the vertices in S?
- Where do we embed them?

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7

9

8

30

30

3

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3

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1

9

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Poly-time alg for longest path on an interval

biconvex graph (idea)

- Embed the vertices in S into P as possible

w(e) is the number at the right-endpoint

3

4

3

7

1

9

2

3

8

8

30

30

3

4

3

7

1

9

2

3

8

8

Poly-time alg for longest path on an interval

biconvex graph (idea)

- Embed the vertices in S into P as possible

w(e) is the number at the right-endpoint

3

4

3

7

1

9

2

3

8

8

30

30

3

4

3

7

1

9

2

3

8

8

Find the maximum weighted matching!!

Open Problems

- Longest Path on an interval graph??
- Combination of DP/Dijkstra and weighted maximum

matching on MPQ-tree representation? - Related to the following open problem?
- Hamiltonian Path with a start point on an

interval graph? Damaschke, 1993. - Extension to
- Longest cycle on some graph classes
- Hamiltonian cycle/path on some graph classes